1. Field of the Invention
The present invention relates to an imaging unit having an imaging optical system and an image sensor that captures light emanating from a photographic object which is passed through the imaging optical system. The imaging optical system and the image sensor are accommodated in a housing of the imaging unit.
2. Description of the Related Art
In recent years, mobile electronic devices which are designed mainly for taking still/moving photographic images, such as digital cameras (still-video cameras) and digital camcorders (motion-video cameras), and other mobile electronic devices which are designed to have the capability of taking such photographic images as a subsidiary function, such as mobile phones equipped with a camera and personal digital assistants (PDAs) equipped with a camera, have become widespread. In these types of mobile electronic devices, it is common to provide the mobile electronic device therein with an imaging unit which is structured such that an image sensor and a photographing optical system for leading light emanating from a photographic object (object to be photographed) to the image sensor are accommodated in a hollow housing of the imaging unit. Additionally, in recent years, the downsizing (slimming down) of the mobile electronic devices has further progressed, so that the demand to also slim down the imaging unit has become stronger.
Japanese Unexamined Patent Publication No. 2008-242446 discloses an example of a conventional imaging unit. This imaging unit is provided with a hollow casing and is further provided in this casing with an imaging optical system and an image sensor. The imaging optical system is provided with an incident-side prism, an exit-side prism and movable lens groups that serve as power-varying lens groups. The incident-side prism and the exit-side prism are installed to be spaced from each other in the lengthwise direction of the casing, and the movable lens groups are positioned between the incident-side prism and the exit-side prism and move forward and rearward along the lengthwise direction of the casing. The traveling direction of light which emanates from a photographic object to be incident on the incident-side prism (which serves as an incident portion of the imaging optical system) changes (i.e., is reflected) at an angle of 90 degrees by the incident-side prism, and thereafter passes through the aforementioned movable lens groups to be incident on the exit-side prism. Thereupon, the traveling direction of the light incident on the exit-side prism again changes (i.e., is reflected) at an angle of 90 degrees by the exit-side prism to be received (captured) by the imaging surface (photosensitive surface) of the image sensor.
Configuring the imaging optical system using two prisms in such a manner makes it possible to guide light emanating from a photographic object to an image sensor while achieving a reduction in the size of the casing in the direction of the thickness thereof (in a direction orthogonal to the lengthwise direction of the casing).
In optical apparatuses, it is often the case that stray light becomes incident on an imaging surface (image-forming surface) from different directions from a normal optical path. If stray light is incident on the imaging surface, a ghost image occurs in the captured (photographed) image, and hence, alight shield frame for shielding stray light traveling toward the imaging surface is often installed. Specifically, in the imaging unit the casing of which is slimmed down as described above, the distance between the inner wall of the casing and the imaging optical system is small, which increases the possibility of stray light which is reflected by the inner wall of the casing or reflected a side of the exit-side prism being directly incident on the imaging surface, and accordingly, the installation of the aforementioned light shield frame is effective in preventing stray light from being incident on the imaging surface.
In general, such a light shield frame is fixed to a movable lens frame which supports a movable lens group (for instance, see Japanese Unexamined Patent Publications Nos. 2003-57721 and 2010-14786).
However, if the light shield frame is fixed to such a movable lens frame, the distance between the light shield frame and the movable lens group in the optical axis direction is constant even if the position of the movable lens group in the optical axis direction is changed, which makes it difficult to locate the light shield frame at an optimum position (for shielding stray light) at each focal length. In addition, the same problem arises also in the case where the light shield frame is supported to be slightly movable relative to the movable lens frame.